1. Technical Field
This disclosure relates generally to reinforcing assemblies for use in structural concrete members.
2. Background of the Related Art
Commercial concrete is a mixture of cement, sand and stone aggregate held together in a rigid structure by the addition of water. So-called “unreinforced” concrete has fairly good resistance to compressive stresses, however, any significant tension tends to break the structure and cause undesirable cracking and separation. To address this problem, concrete is typically “reinforced” by embedding in place (within the rigid structure) a solid member made of a material with high strength in tension. Reinforced concrete structures are available commercially in many shapes and sizes, such as slabs, beams, footings and flat foundations.
Commercial and industrial structural concrete members, even when made with reinforced concrete, are highly susceptible to shear forces that create diagonal tensile forces within them, which can result in structural failure. The cracking and/or breaking caused by these shear forces tend to propagate throughout the concrete structure. In horizontal concrete members (such as slabs, footings and flat foundations), this problem is known as “punching shear failure.” The problem is especially acute in concrete members when supported by columns. In this situation, the concrete member is subject to a concentration of stress in a zone near the column, wherein the column tends to “punch” through the member. The resulting shearing force creates diagonal tension stresses within the supported member. The concrete is particularly vulnerable to these tensile stresses and thus must have reinforcement, such as embedded steel members, to prevent tensile failure, crack propagation, and consequent structural collapse.
The prior art has addressed the problem of punching shear failure by providing assemblies and reinforcing techniques such as described in U.S. Pat. No. 4,406,103. According to this patent, shear reinforcement is provided by a plurality of substantially vertical elongate reinforcing elements (smooth shafts) fixedly attached in spaced horizontal relation to support means, wherein each element is provided (at the upper end) with an enlarged (flange) portion, which serves as an anchor when the reinforcement is embedded within the concrete slab. At the lower end is a flat steel bar, which serves as a base structure and as a lower end anchorage. In a preferred form, the element consists of thin transverse sections. A commercial product incorporating this design is known as the “Stud-Rail” system.
While the Stud-Rail system is well-known and widely-used, it is not an optimal solution to the problem of punching shear failure. As noted above, the system uses smooth shafts for reinforcing, but those shafts have no means to grip the concrete in the area of primary crack formation. Thus, in operational mode, the stresses from diagonal tension must stretch the concrete (in the vulnerable crack zone) away from the center of the slab, accumulating in proportion to the load and the thickness of the concrete until restrained by a flange at the top or the bottom shaft, i.e. near the surface of the top or bottom of the slab thickness. This restraining force places the shaft itself in tension from one end to the other, which causes the shaft to undergo significant strain. In addition, there is a compressive strain in the compressed zones under the flange at the top and bottom of the shafts. To maintain equilibrium, the sum of the top and bottom compression strains under the flanges, and the tensile strain in the shaft of the studs must be equal to the tensile strain in the central zone of the slab thickness. But, because the total thickness of the top and bottom compression zones are roughly equal to the thickness of the tension zone in the center, because the strain in the shafts is additive to the concrete compression strains, and further because the tensile strain in the central concrete zone must equal the sum of the above two strains, the tensile stress in the central concrete zone must be commensurately higher. In response to these high stresses, cracks still are able to form and propagate at relatively low loads. Another deficiency of the Stud-Rail system is that the reinforcement does not start working (albeit inefficiently, for the reasons stated above) until a crack has started; the structure and reinforcing technique do not act as a prophylactic to prevent such cracks in the first instance.
There remains a long-felt need in the art to provide enhanced concrete structure reinforcing assemblies that overcome the deficiencies of current state-of-the-art systems for punching shear reinforcement.